It is known to place valve devices, for example thyristor valves included in plants for series compensation of a.c. lines or for conversion between alternating current and high-voltage direct current, in enclosures suited for outdoor erection, these enclosures being electrically insulated against the surroundings by being located on support insulators of porcelain. It is also known from Japanese patent application JP 63-80763 (A) to pass a light guide to a thyristor valve, located indoors and erected in an insulating manner, by arranging the light guide inside a hollow porcelain insulator filled with SF6 gas. By supplying the valve device with control signals and other media via a separate insulator, possibilities are opened for separately designing both this separate insulator and the support insulators in an optimum manner in view of their respective functions.
An insulator adapted for supply of control signals must in general be designed with a larger diameter than the support insulators, and particularly in those cases where it is additionally utilized for supply and/or removal of temperature-influencing media for the valve device and/or the insulating medium of the valve device. A porcelain insulator for this purpose is therefore usually heavy and difficult to manufacture, particularly in the lengths which may be required for high-voltage plants. Particularly in plants for high-voltage direct current, it must also be taken into consideration that the risk of flashover across the insulators is greatly depending on deposits on the insulator surfaces, and therefore it is desirable with a hydrophobic insulator surface in the case of outdoor location. The internal dielectric strength of the insulator is negatively influenced by the channels and conductors which are placed therein for the purposes stated above, so it is important that the risk of condensation and the ensuing risks of flashover are minimized by control of the climate in the interior of the insulator.